Substantially pure magnesium metal can be electrolytically produced from magnesium chloride with evolution of chlorine gas. However, if hydrated magnesium chloride is used as the feed to the electrolytic cell, the efficiency of the cell significantly decreases over a short period of time as oxides of magnesium are formed which corrode the electrodes and produce a sludge which must be periodically removed from the cell. Magnesium chloride feeds also typically contain impurities such as hydrocarbons, boron and other metal salts which also substantially reduce the efficiency of the electrolytic cell. Accordingly, it is desirable to produce substantially pure anhydrous magnesium chloride which is suitable for electrolytic production of magnesium metal.
Depending upon the temperature, magnesium chloride isolated from aqueous solutions contains a variety of numbers of molecules of water of crystallisation. Hydrated forms of magnesium chloride can be dehydrated to some extent by heating. However, hydrated magnesium chlorides tend to melt in their own water of crystallisation to form a thickened partially dehydrated product which is very difficult to further dehydrate by heating. Further, it is not possible to fully dehydrate magnesium chloride by heating in air because magnesium chlorides containing less than two waters of crystallisation undergo hydrolytic decomposition with evolution of hydrogen chloride rather than dehydration. Accordingly, alternative approaches have been proposed for the production of anhydrous magnesium chloride.
Anhydrous magnesium chloride can be made by direct chlorination of magnesium and by drying with hydrogen chloride gas. The former process is clearly not a viable method for producing anhydrous magnesium chloride for use in the electrolytic production of magnesium metal. In the latter process, hydrated magnesium chloride is made into pellets which are placed in a column and are purged by hot hydrogen chloride to remove all traces of water. This latter process has not proven to be very efficient as there are generally hydrated forms of magnesium chloride present at the end of the process which may, in subsequent uses, convert to oxides of magnesium which are not desired. Further, this process requires the use of large quantities of hydrogen chloride gas which has many problems associated with its storage and use.
An alternative approach to the production of anhydrous magnesium chloride has involved forming a solution of hydrated magnesium chloride in a solvent, removing water from the solution, forming a magnesium chloride complex by reaction of the water-free solution with a precipitating agent and heating the magnesium chloride complex to produce anhydrous magnesium chloride. A number of variations of this general approach have been proposed in patent literature over the years with a common feature of the variations being the use of ammonia as the precipitating agent. Such processes are hereafter referred to as ammoniation processes. Various problems have been associated with ammoniation processes and the present applicants are not aware of anhydrous magnesium chloride having ever been commercially produced by an ammoniation process.
The desired magnesium chloride complex which is heated to produce anhydrous magnesium chloride in an ammoniation process is magnesium chloride hexammoniate (MgCl.sub.2.6NH.sub.3). Where ethylene glycol is used as the solvent for forming the solution of hydrated magnesium chloride, the present inventors have ascertained that magnesium chloride glycollate compounds can be formed during an ammoniation process in addition to or in lieu of magnesium chloride hexammoniate. Magnesium chloride glycollate compounds include magnesium chloride triglycollate (MgCl.sub.2.3(HOCH.sub.2.CH.sub.2 OH)) and magnesium chloride biglycollate biammoniate (MgCl.sub.2.2(HOCH.sub.2.CH.sub.2 OH).2NH.sub.3). The present inventors have identified characteristic X-Ray Diffraction (XRD) patterns and Fourier Transform Infrared (FTIR) spectra for magnesium chloride glycollate compounds by which the presence of such compounds can be identified. Details are provided in Tables 1-4. Magnesium chloride glycollate compounds are undesirable in an ammoniation process as they are believed to decompose on heating to form oxygen containing compounds which contaminate the is desired anhydrous magnesium chloride product. The introduction of oxygen into an electrolytic cell into which the product is fed in the production of magnesium metal reduces both the life of carbon anodes typically used in such cells and the efficiency of the cell.
U.S. Pat. No. 2,381,995, which was filed in 1942 and assigned to The M W Kellogg Company, teaches an ammoniation process with a preference for isoamyl alcohol as the solvent used to form the solution of hydrated magnesium chloride and ammonia as the precipitating agent with magnesium chloride hexammoniate identified as the magnesium chloride complex which is heated to form anhydrous magnesium chloride. U.S. Pat. No. 2,381,995 also teaches thorough intermixing of the water-free solution of magnesium chloride and the ammonia prior to their introduction to a cooler where the magnesium chloride hexammoniate is said to precipitate.
U.S. Pat. No. 3,966,888, which was filed in 1975 and assigned to the Nalco Chemical Company, also teaches an ammoniation process. In discussing U.S. Pat. No. 2,381,995, U.S. Pat. No. 3,966,888 states that U.S. Pat. No. 2,381,995:
"relies upon dissolving a hydrated magnesium chloride in a monohydroxy saturated aliphatic alcohol. This solution is then heated for a period of time sufficient to drive off the water present. The alleged water-free solution is treated with ammonia to precipitate a magnesium chloride ammonia complex which is then separated from the alcohol and heat-treated to drive the ammonia from the complex.
The difficulties experienced in actually practicing the techniques of U.S. Pat. No. 2,381,995 readily indicate to one skilled in the art that its method is inherently incapable of being adapted to large scale commercial operations.
In the first instance, when the alcohol solution of the hydrated magnesium chloride is heated to remove water therefrom, it is impossible to remove the water at about the boiling point of the alcohol employed. This is particularly true when isoamyl alcohol is used as the solvent for the hydrated magnesium chloride. Thus, the magnesium chloride is not fully dehydrated. When the alleged water-free magnesium chloride is ammonia precipitated from the alcohol as represented by the patentee in U.S. Pat. No. 2,381,995, a dense, wax-like precipitate occurs which contains large quantities of entrained alcohol. The density and wax-like character of the precipitate renders it incapable of being handled by commercial equipment to free the precipitate of entrained solvent. Thus, it is impossible to further process the precipitate without substantial losses of the solvent taking place during the ammonia removal phase of the process."
U.S. Pat. No. 3,966,888 broadly claims a "method of preparing anhydrous magnesium chloride from magnesium chloride hydrates which comprises the steps of:
A) dissolving a magnesium chloride hydrate in ethylene glycol to form an ethylene glycol magnesium chloride hydrate solution; PA1 B) heating the ethylene glycol magnesium chloride hydrate solution to a temperature and for a period of time sufficient to remove all the water therefrom to produce an ethylene glycol anhydrous magnesium chloride solution; PA1 C) treating the ethylene glycol anhydrous magnesium chloride solution with ammonia to form a magnesium chloride ammonia complex which is insoluble in the ethylene glycol, with the temperature of the ethylene glycol magnesium chloride solution being within the range of between -15.degree. to 50.degree. C.; PA1 D) separating the magnesium chloride ammonia complex from the ethylene glycol; PA1 E) washing the magnesium chloride ammonia complex with a polar solvent having a lower boiling point than ethylene glycol to remove any ethylene glycol entrained in the magnesium chloride ammonia complex; PA1 F) heating the magnesium chloride ammonia complex to a temperature and for a period of time sufficient to drive off the ammonia, thereby forming anhydrous magnesium chloride; and then, PA1 G) recovering anhydrous magnesium chloride which has a magnesium oxide content less than 0.8% by weight." PA1 a) the ethylene glycol anhydrous magnesium chloride solution that has been cooled to between -15.degree. and 50.degree. C. is treated with at least 6 moles of ammonia, based on the magnesium chloride present in the ethylene glycol; and PA1 b) the ethylene glycol anhydrous magnesium chloride solution is cooled to between 0.degree. and 25.degree. C. prior to ammonia addition thereto. PA1 (a) forming an alcohol magnesium chloride solution by admixing hydrated magnesium chloride with an alcohol which is miscible with water; PA1 (b) dehydrating the alcohol magnesium chloride solution to form a dehydrated alcohol magnesium chloride solution; PA1 (c) forming a precipitate comprising magnesium chloride hexammoniate by introducing the dehydrated alcohol magnesium chloride solution and ammonia into a reaction vessel containing a non-aqueous solution having an ammonia content of greater than 7% by weight; PA1 (d) recovering the precipitate from the reaction vessel; PA1 (e) washing the recovered precipitate with a washing solvent to form a washed precipitate; and PA1 (f) heating the washed precipitate to form substantially anhydrous magnesium chloride. PA1 (i) removing any ammonia or polar solvent, such as methanol, from alcohol from the reaction vessel; PA1 (ii) mixing a soluble magnesium bicarbonate solution with the alcohol from step (i) to form a mixture of magnesium bicarbonate and alcohol; PA1 (iii) heating the mixture of magnesium bicarbonate and alcohol to form a precipitate of calcium carbonate; and PA1 (iv) separating the calcium carbonate precipitate. PA1 (i) removing any ammonia or polar solvent, such as methanol, from the alcohol; and PA1 (ii) continuously adding the alcohol from step (i) to the top of a stripping column in which steam is added continuously to the bottom of the column with a salt/water solution which is substantially free of alcohol being withdrawn from the bottom of the column and a vapour stream of alcohol and water containing no salts being withdrawn from the top of the column. PA1 (1) any alcohol removed from the alcohol magnesium chloride solution by dehydration in step (b), PA1 (2) the washings resulting from washing the recovered precipitate, and PA1 (3) ammonia and any alcohol removed from the washed precipitate in step (f).
U.S. Pat. No. 3,966,888 also claims the above method in which, in relation to step C), it is more narrowly specified that
In describing step C), U.S. Pat. No. 3,966,888 teaches that:
"The anhydrous ethylene glycol magnesium chloride solution is then cooled to about -15.degree.-50.degree. C. and, preferably, within the range of 0.degree.-25.degree. C. At this point the solution is treated with anhydrous ammonia to provide at least 6 moles of ammonia and, preferably, at least 9 moles of ammonia per mole of magnesium chloride present in the ethylene glycol solution. The ammonia addition can be relatively rapid although in small-scale laboratory preparations, the ammonia addition should take place over a period of time ranging between 1-2 hours.
It was found that by cooling the magnesium chloride ethylene glycol solution to the temperature indicated that the ammonia is more soluble therein and that a precipitate does not form until at least 6 moles of the ammonia have been added. After most of the ammonia is added to the glycol, a fine, white, grainy precipitate begins to form which is a water-free ammonia complex of the magnesium chloride."
In exemplifying step C), U.S. Pat. No. 3,966,888 teaches that the ethylene glycol anhydrous magnesium chloride solution is cooled to 15.degree. C. and approximately 9 moles of anhydrous ammonia is added to the cooled solution over a period of one hour with a precipitate beginning to form after half an hour of ammonia addition.
U.S. Pat. No. 3,966,888 also teaches a preference for the ethylene glycol anhydrous magnesium chloride solution to contain 8-12% by weight magnesium chloride.
A paper entitled "A New Economical Process for Making Anhydrous Magnesium Chloride" by Dr Ronald J Allain of the Nalco Chemical Company (hereafter referred to as the Allain paper) was published in 1980 and discusses an ammoniation process based on the teaching of U.S. Pat. No. 3,966,888. In relation to step C) of U.S. Pat. No. 3,966,888, the Allain paper teaches that in order to separate magnesium chloride from the anhydrous magnesium chloride in ethylene glycol solution, gaseous anhydrous ammonia is bubbled through the solution. The ammonia is said to immediately dissolve, first saturating the ethylene glycol and then forming a magnesium chloride hexammoniate precipitate. The paper specifies that it is most convenient to do the ammoniation in a simple stirred tank arrangement with the ammonia introduced slightly above atmospheric pressure. The reaction is said to be essentially instantaneous with no ammonia escaping through the vent under the conditions employed with cooling water used to remove the majority of the heat liberated. The paper specifies that in practice, the reactor is allowed to heat up to 70.degree. C. and is then chilled during the course of the reaction to the final temperature of 15-30.degree. C. Accordingly, the teaching of the Allain paper is essentially the same as step C) of U.S. Pat. No. 3,966,888, ie. the ethylene glycol magnesium chloride solution is treated with ammonia by placing the ethylene glycol magnesium chloride solution in a vessel and thereafter adding ammonia to the vessel. One difference between the Allain paper and U.S. Pat. No. 3,966,888 is that the Allain paper refers to cooling of the reactor from 70.degree. C. to 15-30.degree. C. during the course of the ammonia addition; whereas, U.S. Pat. No. 3,966,888 requires the temperature of the ethylene glycol magnesium chloride solution to be within the range of -15-50.degree. C. prior to addition of anhydrous ammonia.
The present inventors attempted to produce anhydrous magnesium chloride by following the teachings of U.S. Pat. No. 3,966,888 and the Allain paper with details being provided in a Comparative Example. To that end, an ethylene glycol anhydrous magnesium chloride solution was prepared according to U.S. Pat. No. 3,966,888. The solution, which contained by weight magnesium chloride, was cooled to 15-20.degree. C. and attempts were made to add 6-9 moles of anhydrous ammonia to the cooled solution. However, it was found that the cooled solution was much too viscous to allow the ready injection and dispersion of anhydrous ammonia. The present inventors found that difficulties resulting from the viscosity of the cooled solution were alleviated if the temperature of the ethylene glycol anhydrous magnesium chloride solution was in the order of 70.degree. C. prior to addition of ammonia and that precipitate yield was increased if the temperature following ammonia addition was reduced to 15.degree. C. However, the resultant precipitate was found not to be substantially magnesium chloride hexammoniate as desired but to contain significant amounts of undesirable magnesium chloride glycollate compounds. Accordingly, it is believed that the teachings of U.S. Pat. No. 3,966,888 and the Allain paper are inherently incapable of being used in commercial anhydrous magnesium chloride production.